It sounds like a mistake to me. The techs take apart dozens, if not hundreds of Macs a day down at the depot. They probably just forgot how much memory was supposed to be in, or temporarily confused yours with someone else's, or put memory in for testing and forgot to take it out.

You can try to return the extra Apple memory I suppose...I don't know if they'd accept it...or just sell your surplus on Craigslist.